1. Field of the Invention
The invention relates to sonar transducers and in particular to elliptical shell flextensional transducers as described in U.S. Pat. No. 4,462,093 which are used to generate and radiate high power acoustic energy at low frequencies, typically in the range 200-3000 Hz.
2. Discussion of Prior Art
The construction of an elliptical shell flextensional transducer comprises fitting a piezo-electric stack (or stacks) along the major axis between opposing internal walls of an elliptical flexural shell in cylindrical form. Each stack consists of a number of piezo-electric plates between which are sandwiched metal electrodes, these in turn being connected in parallel. When an alternating voltage is applied to the electrodes a vibration is generated along the length of each stack. This vibration is transmitted to the shell and leads to an amplified out-of-phase vibration along the minor axis of the shell which provides the principal motive force for this sonar transducer.
Conversely, the transducer can be used in a passive mode in which received vibrations induce minor axis vibrations in the elliptical shell which in turn lead to electrical signals generated by the piezo-electric stacks.
The elliptical shells are commonly made of filament-wound glass reinforced plastics ("GRP"), the filament being wound around a suitable mandrel. The elliptical shell is then compressed along its minor axis by means of a press to permit assembly of the piezo-electric stacks along the major axis such that on removing the compressive force along the minor axis a residual tension remains in the shell to retain the stacks and apply a predetermined stress to the stacks. The stress applied to the piezo-electric stack must be set to a precise value, since when the transducer is deployed into water the increasing hydrostatic pressure with depth reduces the stress on the stack until a limit is reached beyond which the elliptical shell transducer cannot be driven without damage. To achieve the desired stress in the shell the piezo-electric stack output charge can be monitored at discrete conditions after placing shims of different discrete thickness at the end of the stack or continuously by using appropriately tapered wedges such as described in our copending patent application Ser. No. 8,606,745.
Flextensional transducers are normally sealed by means of end plates, however because they are capable of high power operation and thus the large amplitude flexing of the elliptical shell which occurs creates difficulties in water-tight sealing between the shell and end-plates since the sealing must be effective without limiting shell movement.
In order to operate there must be a pre-stress load applied by the elliptical shell to the transducer stacks. Operation over a wide range of pressure-depths requires that some form of pressure-balancing arrangements is provided.
Conventional pressure compensation or balancing systems have a number of operational disadvantages. The most common types of pressure balancing systems are air filled bladders and scuba type systems of which the latter use bottled compressed air coupled to a divers pressure balanced valve. The bladder method is severely limited as the volume of air in the cavity of the transducer is inversely proportional to the external hydrostatic pressure. The resulting reduction of the available swept volume for the active surface progressively lowers operating efficiency as the hydrostatic pressure is increased. The scuba system is a large and often relatively heavy appendage to a sonar transducer. In operation it can use large quantities of air if frequent changes in operating depth are required or if there are large unwanted depth excursions due to the effects of ocean swell on the deployment platform.
In a conventional design of flextensional transducer the dimensions of the shell are calculated to utilize the first and sometimes other flexural modes of vibration along the entire length of the oval cylinder. The shell has therefore a single resonance frequency and a finite bandwidth associated with each flexural mode.